Desugaring

Desugaring is the process in programming language implementation where syntactic sugar—syntax that is more convenient for humans but not essential to express computations—is translated into more primitive language constructs. The desugared form uses a small core language that the compiler or interpreter already handles directly.

The purpose of desugaring is to reduce the surface area that the rest of the system must support and to improve portability, optimization, and reasoning about code. It also helps unify features by expressing them in terms of a common core.

In practice, desugaring is performed as part of the compilation or interpretation pipeline, typically after parsing and possibly before semantic analysis. It may be a source-to-source transformation or occur at lowering to an intermediate representation. The transformation should preserve semantics under the language's evaluation model.

Examples illustrate its role. JavaScript demonstrates many cases, such as for...of loops desugaring to calls to the iterator protocol, async/await desugaring to promise-based state machines, and arrow functions desugared to function expressions. Haskell desugars do-notation and list comprehensions into monadic binds and concatenation of lists. In languages with macro facilities, sugar can also be implemented via hygienic macros, which expand into core constructs.

Limitations exist. Desugaring assumes certain evaluation order and side-effect behavior; otherwise, semantics can change if the target core language differs. Some sugars obscure complexity that would be visible in the core language, so performance implications may arise after desugaring.

See also: syntactic sugar; macros.